1. Field of the Invention
This invention relates generally to a low current blow trim fuse associated with any integrated circuit that relies on fuses for trimming, and more particularly, to a structure of a trim fuse capable of being blown out easily and reliably at a low current and its associated method of manufacture.
2. Description of the Prior Art
Any integrated circuit that relies on fuses for trimming, such as precision references, voltage regulators, op-amps and the like, is subjected to electrical stresses during trimming processes, that adversely impacts device reliability. These trimming processes associated with such devices generate currents necessary to program the trimming fuses. In view of the foregoing, a need exists for a technique that will reduce the amount of current needed to program trimming fuses to reduce the electrical stress that a device will undergo during trimming operations and to also reduce the stresses on probe cards and associated test circuitry, thus extending their useful life.
The minimum amount of current necessary to blow a trim fuse is limited by the cross-sectional area of the metal (or polycrystal silicon (poly)). In some processes, the minimum metal (or poly) width is limited by lithography and process variations that can be quite large. The current required to program the trim fuse(s) can therefore also be very large (i.e. greater than one Ampere for metal fuses).
Known solutions to reducing the foregoing electrical stresses associated with trim fuses have concentrated on thinning the width of the metal (or poly) fuse, or putting corners in the fuse. One such solution is disclosed in U.S. Pat. No. 4,984,054, entitled Electric Fuse For A Redundancy Circuit, issued Jan. 8, 1991 to Yamada et al. These known solutions only address the electrical stress problem to the extent of the minimum reliable metal (or poly) width that can be drawn in a given process. Since the cross-sectional area is most important in determining the amount of current required to program a trim fuse, it is desirable to provide a technique that will enable a designer to minimize the trim fuse cross section in two dimensions, rather than only one, without adding process steps.
The present invention is directed to a low current blow trim fuse structure and method of forming the trim fuse structure. Oxide steps are placed beneath the trim fuse during prior process steps. The oxide steps will cause the metal (or poly) to thin at the point where the metal (or poly) transitions the step, and thus will reduce its cross-sectional area and current carrying capability, making it easier to program the fuse. The oxide steps will serve a further purpose in that, to some extent, it will thermally isolate the trim fuse, thereby causing local heating, making the fuse easier to blow.
In one aspect of the invention, a low current blow trim fuse is provided having a reduced cross-sectional area in two dimensions to reduce its current carrying capacity, without adding process steps.
In another aspect of the invention, a low current blow trim fuse is provided to minimize its reliance on lithography and process variations.
In still another aspect of the invention, a low current blow trim fuse is provided having thermal isolation characteristics that enhance localized heating of the fuse.